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  #1  
Old 08-24-2008, 10:35 PM
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DIY w/pix -- W124 fuel sender disassembly/cleaning/testing

This DIY is based on a W124 car but the W123 cars have a similar fuel gauge sender. It applies to gassers as well as diesels.

When I removed the fuel gauge sender from my '87 300D Turbo (W124, OM603) to clean it, I found the plastic top had some serious cracks so I decided to replace it. After discovering the price of a new one ($100+) I decided to check the two 300Es at my local Pick and Pull. First question was of course "are they the same diesel/gasser" and that was quickly answered "yes" in this thread. [Thanks!]

Removing the W124 sedan fuel sender is done from inside the trunk, not through the first-aid compartment as in the W123. The fuel tank must be no more than 3/4 full or you will turn your trunk into a fuel bathtub. If you're small enough you can lie in the trunk and work. If not, have the kids do the job!

The fuel tank is behind the fabric partition at the rear (front?) of the trunk. Remove the partition and you will see the fuel tank with the sender at top center. Pull the electrical connector off and remove the big nut with large pipe pliers. While the big nut is off, check its two o-rings and replace them if they look old/hard/damaged. Once the nut is off, you can remove the sender, tilting it as necessary and pausing part way to let residual fuel dribble into the tank. If you are in a hurry, have a rag or pan ready to catch the spill. Now take the sender to your workbench and get ready to take it apart.

At the junkyard I pulled two senders -- one from an '88 300E and one from an '89. The '88 was cleaner so I bought it (about $25). I decided to take pictures to help others who might never have opened one of these up for cleaning or repair. It's not difficult.






The diesel and gasser parts are identical.




Mine had some bad cracks [140,000 miles on D2 and about 8,000 on B100]. Are these due to age, heat, diesel fuel? The gasser's sender was cleaner, no question. I suppose that is to be expected.




The black plastic bottom is held on by a funny nut.




Loosen the nut with a pair of miniature needle-nose pliers or make a special-purpose driver by grinding a slot in a small screwdriver.




Once the nut is loose the bottom will come off.




Now the insides can come apart for cleaning or repair. Be careful of the two wires that run from top to bottom. They aren't terribly fragile but could be broken in a moment of carelessness. They can't be seen in this picture -- too fine.

I used brake cleaner to spray off the components and used fine steel wool to polish the central rod on which the float rides (make sure you don't leave any threads of steel after cleaning up). Check the two pins that switch on the "low fuel" light and clean them and the matching contacts on the float if they are dirty or corroded. Use VERY fine sandpaper or steel wool and work slowly and carefully. Clean off any abrasive particles left over.

Test the sender with an ohmmeter (look at the picture labeled "Electrically identical"). The resistance of the sender varies from zero (tank full, float at top of sender) to about 90 ohms (tank empty, float at bottom of sender). The common lead is pin 3 of the connector (brown wire), lower left pin in the picture. The sender is pin 4 (blue/black wire), lower right pin in the picture. The third terminal is for the "low fuel" light. It is pin 3 in the electrical connector (blue/green wire), upper right next to the plastic pin in the picture. The low fuel switch is "open" (infinite resistance) until the float gets all the way to the bottom; then it is closed and has zero resistance.

Put the sender back together carefully -- don't snag the wires with the case -- and put it back in the fuel tank. Screw on the big nut, tighten it with the pipe pliers, put the electrical connector back, replace the partition. You're finished.

Jeremy
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"Buster" in the '95

Our all-biodiesel family
1995 E300D (W124) . .239,000 miles My car
1996 E300D (W210) . .313,000 miles Wife's car
Santa Rosa population 170,685 (2012)
Total. . . . . . . . . . . . 722,685
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970
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  #2  
Old 08-24-2008, 10:51 PM
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Nice Writeup

Clear Pictures
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  #3  
Old 08-25-2008, 11:25 PM
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Break a wire?

If one (or both) of the wires in your sender have broken, here's what you need to know to fix them.

One of the wires is nichrome resistance wire or something similar. In the "Disassembled" picture (above), it's the upper wire, the one closest to you in the picture (both wires are invisible in the picture -- too small). The lower wire, next to the fabric on which the sender is lying, is ordinary wire.

The two wires cannot be switched. The "ordinary" wire is part of the low-fuel light circuit and needs to be low resistance or the light won't work. The resistance wire makes the fuel gauge work.

The resistance wire measures 7.5 Ohms per inch (3 Ohms per centimeter). It is 0.0015 inch (0.04 mm) in diameter or 46 gauge. You may be able to buy nichrome resistance wire at a good electronics store.

The other wire is about 0.004 inch (0.10 mm) in diameter or 38 gauge. Any fine-gauge wire will work. One forum member took one strand from a stranded wire and made his sender work.

Jeremy
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"Buster" in the '95

Our all-biodiesel family
1995 E300D (W124) . .239,000 miles My car
1996 E300D (W210) . .313,000 miles Wife's car
Santa Rosa population 170,685 (2012)
Total. . . . . . . . . . . . 722,685
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970
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  #4  
Old 08-26-2008, 01:13 AM
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jeremy,

you are always adding somethign to my list thanks for the great writeup. even better pics!

bob
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  #5  
Old 05-11-2010, 04:30 PM
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Is this a fix for a bouncing fuel gauge? That is my issue. Or could there be a loose ground elsewhere?

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Old 05-11-2010, 04:37 PM
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Quote:
Originally Posted by freesoul View Post
Is this a fix for a bouncing fuel gauge?
... that could be it - it was the fix in my case... my fuel gauge was working ok until it got down to about 1/4 tank and then it would 'bounce around' uncontrollably... after removal, disassembly and cleaning it's working fine now. Check out the clarity of the fuel in the tank while you're at it - remove a small portion of the fuel into a clear plastic bottle - mine was black... and so another project began...
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Old 05-11-2010, 05:01 PM
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Quote:
Originally Posted by MercFan View Post
... that could be it - it was the fix in my case... my fuel gauge was working ok until it got down to about 1/4 tank and then it would 'bounce around' uncontrollably... after removal, disassembly and cleaning it's working fine now. Check out the clarity of the fuel in the tank while you're at it - remove a small portion of the fuel into a clear plastic bottle - mine was black... and so another project began...
Thanks, will check it out. I make and run my own biodiesel so I'm not real worried about my fuel quality
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Old 05-11-2010, 05:47 PM
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Quote:
Originally Posted by freesoul View Post
.. I make and run my own biodiesel so I'm not real worried about my fuel quality
me too... but not in the SDL (just yet)... good luck out there.
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  #9  
Old 06-23-2010, 10:42 PM
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So glad I found this. I put fuel system cleaner in my tank a few days ago, for the first time since I've owned the car. Well last night, when my tank got to 1/4 full, the needle started bouncing around. I assume that the fuel treatment loosened up some gunk and now it's affected my sender. I will need to take it apart and check it.

Now I need to see if I can find a write-up like this for my 450sl.
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  #10  
Old 09-07-2010, 10:01 AM
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Thank you. I needed an excuse to order a 42mm socket
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  #11  
Old 12-13-2010, 10:03 PM
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When buying the socket make sure you get it on a 1/2 inch drive and not larger with an adapter,because this causes a lot of space constriction and hard if not impossible to get the sender loose.
My sender on the w124 300d was bad after previous owner ran wvo for years in car, i had a spair form a 300d,it is in the car but i do not see the needle dropping a whole lot,maybe i get great mileage or maybe its stuck?
You are sure the 300d and 300d are identical right??
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  #12  
Old 12-13-2010, 10:33 PM
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Quote:
Originally Posted by europower View Post
. . . You are sure the 300d and 300d are identical right??
If you mean 300E and 300D, yes, they are the same part number. Check in EPC if you wish. I have a 300E sender in my 300D's fuel tank and it works perfectly. From the several that I looked at in the local wrecking yard, the gasser senders were a little cleaner than the diesel senders.
__________________

"Buster" in the '95

Our all-biodiesel family
1995 E300D (W124) . .239,000 miles My car
1996 E300D (W210) . .313,000 miles Wife's car
Santa Rosa population 170,685 (2012)
Total. . . . . . . . . . . . 722,685
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970
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  #13  
Old 06-24-2013, 09:32 AM
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jeremy, fantastic explanation. i wonder if you might know if this might solve my problem. i have an e200 and the other day i put 50 euros in it, first time in a long time, and gauge went upto 3/4. on taking off, gauge fluctuated up and down, but never going below 3/4 and the reserve light would come on at speed. now it appears stuck at 3/4, obviously until i turn the engine off. is it possible that something got in the sending unit while filling up and that has caused the gauge to stick and the light to come on intermittently?

thanks in anticipation, and hopefully i'll be able to learn and contribute here in the future

cheers dave
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  #14  
Old 06-24-2013, 09:42 AM
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Only way to tell is to pull the sender and disassemble. Sounds like yours is dirty. Be gentle, the wires inside are very small diameter.

Jeremy


Quote:
Originally Posted by davehare View Post
jeremy, fantastic explanation. i wonder if you might know if this might solve my problem. i have an e200 and the other day i put 50 euros in it, first time in a long time, and gauge went upto 3/4. on taking off, gauge fluctuated up and down, but never going below 3/4 and the reserve light would come on at speed. now it appears stuck at 3/4, obviously until i turn the engine off. is it possible that something got in the sending unit while filling up and that has caused the gauge to stick and the light to come on intermittently?

thanks in anticipation, and hopefully i'll be able to learn and contribute here in the future

cheers dave
__________________

"Buster" in the '95

Our all-biodiesel family
1995 E300D (W124) . .239,000 miles My car
1996 E300D (W210) . .313,000 miles Wife's car
Santa Rosa population 170,685 (2012)
Total. . . . . . . . . . . . 722,685
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970
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  #15  
Old 06-24-2013, 09:45 AM
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ill buy a spanner and give it a go, thanks

cheers dave
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